Lamp ballasts have generally been tested by measuring the voltage at the lamp socket. This kind of testing is satisfactory when checking ballasts used with mercury vapor or metal halide lamps but does not work well with high pressure sodium (HPS) lamp ballasts. When checking HPS lamp ballasts it is necessary to connect a low pass filter at the input of the voltmeter to prevent the meter from being damaged by very high voltage peaks which can occur if the starter circuit is good.
Then, if the initial voltage reading shows that the ballast is good, another probe must be used to determine the voltage produced by the starter circuit. Thus, the field electrician needs to carry at least two special test instruments. Furthermore, the circuitry for the starter probe is very complex and densely packed in currently available instruments.